The relevance of ambipolar diffusion for neutron star evolution

We study ambipolar diffusion in strongly magnetized neutron stars, with special focus on the effects of neutrino reaction rates and the impact of a superfluid/superconducting transition in the neutron star core. For axisymmetric magnetic field configurations, we determine the deviation from β-equilibrium induced by the magnetic force and calculate the velocity of the slow, quasi-stationary, ambipolar drift. We study the temperature dependence of the velocity pattern and clearly identify the transition to a predominantly solenoidal flow. For stars without superconducting/superfluid constituents and with a mixed poloidal–toroidal magnetic field of typical magnetar strength, we find that ambipolar diffusion proceeds fast enough to have a significant impact on the magnetic field evolution only at low core temperatures, T ≲ 1–2 × 108 K. The ambipolar diffusion time-scale becomes appreciably shorter when fast neutrino reactions are present, because the possibility to balance part of the magnetic force with pressure gradients is reduced. We also find short ambipolar diffusion time-scales in the case of superconducting cores for T ≲ 109 K, due to the reduced interaction between protons and neutrons. In the most favourable scenario, with fast neutrino reactions and superconducting cores, ambipolar diffusion results in advection velocities of several km kyr−1. This velocity can substantially reorganize magnetic fields in magnetar cores, in a way which can only be confirmed by dynamical simulations.AP acknowledges support from the European Union under the Marie Sklodowska Curie Actions Individual Fellowship, grant agreement no. 656370. This work is supported in part by the Spanish MINECO grants AYA2013-42184-P and AYA2015-66899-C2-2-P, and by the New Compstar COST action MP1304